UC Irvine

Excitotoxic mechanisms contribute to the degeneration of hippocampal pyramidal neurons after recurrent seizures and brain ischemia. However, susceptibility differs, with CA1 neurons degenerating preferentially after global ischemia and CA3 neurons after limbic seizures. Whereas most studies address contributions of excitotoxic Ca²⁺ entry, it is apparent that Zn²⁺ also contributes, reflecting accumulation in neurons either after synaptic release and entry through postsynaptic channels or upon mobilization from intracellular Zn²⁺-binding proteins such as metallothionein-III (MT-III). Using mouse hippocampal slices to study acute oxygen glucose deprivation (OGD)-triggered neurodegeneration, we found evidence for early contributions of excitotoxic Ca²⁺ and Zn²⁺ accumulation in both CA1 and CA3, as indicated by the ability of Zn²⁺ chelators or Ca²⁺ entry blockers to delay pyramidal neuronal death in both regions. However, using knock-out animals (of MT-III and vesicular Zn²⁺ transporter, ZnT3) and channel blockers revealed substantial differences in relevant Zn²⁺ sources, with critical contributions of presynaptic release and its permeation through Ca²⁺- (and Zn²⁺)-permeable AMPA channels in CA3 and Zn²⁺ mobilization from MT-III predominating in CA1. To assess the consequences of the intracellular Zn²⁺ accumulation, we used OGD exposures slightly shorter than those causing acute neuronal death; under these conditions, cytosolic Zn²⁺ rises persisted for 10-30 min after OGD, followed by recovery over ∼40-60 min. Furthermore, the recovery appeared to be accompanied by mitochondrial Zn²⁺ accumulation (via the mitochondrial Ca²⁺ uniporter MCU) in CA1 but not in CA3 neurons and was markedly diminished in MT-III knock-outs, suggesting that it depended upon Zn²⁺ mobilization from this protein.

Significance statement

The basis for the differential vulnerabilities of CA1 versus CA3 pyramidal neurons is unclear. The present study of events during and after acute oxygen glucose deprivation highlights a possible important difference, with rapid synaptic entry of Ca²⁺ and Zn²⁺ contributing more in CA3, but with delayed and long-lasting accumulation of Zn²⁺ within mitochondria occurring in CA1 but not CA3 pyramidal neurons. These data may be consistent with observations of prominent mitochondrial dysfunction as a critical early event in the delayed degeneration of CA1 neurons after ischemia and support a hypothesis that mitochondrial Zn²⁺ accumulation in the early reperfusion period may be a critical and targetable upstream event in the injury cascade.